Sleep is a physiological process that is commonly described in sleep stages deduced from measurements of brain activity and transition between these stages. One may for instance distinguish the stages of wake, light sleep, deep sleep, and REM (Rapid Eye Movements) sleep. In these stages, the arousal thresholds may differ (i.e. it may be more or less difficult to awaken a person by a certain stimulus), and when a person is made to awaken during these stages the effects on the person's experience of well-being may differ. Especially when a person is made to awaken while in deep sleep (slow-wave sleep, SWS), he may feel groggy and sleepy for some time, and it may take relatively long before this person is actually awake and feeling well. This phenomenon is also known as “sleep inertia”.
Sleep inertia is a very well-known problem and is a number one cause of stress during the morning rituals. The effect of sleep inertia is especially severe for persons of so-called late chrono type (also indicated as owls) when compared to those of so-called early chrono type (also indicated as larks). Owls suffer more for two scientifically confirmed reasons:    1) their circadian phase is not optimal for optimal performance in the morning;    2) sleep duration on workdays is shortened because of late sleep onset and early wake times, resulting in sleep deprivations; sleep deprivation in turn is known to result in an increased proportion of deep sleep, which increases the chances that this person needs to wake up from a deeper sleep.
The classical wake-up device is an alarm clock. When he goes to sleep (or at an earlier point in time), the user sets a wake-up time. When the actual time of day becomes equal to the pre-set wake-up time, the alarm clock generates an audible stimulus (sound). When the user is in deep sleep at that moment, chances are that he will suffer from sleep inertia to some degree. Modern prior art wake-up devices such as Philips Wake-Up Light already reduce this problem of sleep inertia by means of introducing an artificial dawn effect.